Photodynamic Therapy, PDT, is undergoing extensive basic, preclinical and clinical development and testing for the localization and treatment of cancer. The light source used for activating the drug in preclinical and clinical PDT trials has been the argon pumped dye laser. The dye laser is tuneable, but its reliability is low. The overall efficiency of the present PDT argon/dye laser systems is extremely low, which makes it unreliable. This has led to frustration for the PDT researchers and clinicians. It is proposed to develop a solid state laser to meet the requirement of an efficient and reliable light source to match the need of some of the new photosensitizing drugs being developed and tested for PDT. These new drugs, metallopurpurins and some chlorin compounds, have relatively broad absorption bands in the 660 (+/-20)nm region of the red spectrum which makes them potentially good clinical PDT drugs. Using a second harmonic generating crystal, KTP or BBO, it is possible to generate 659nm laser light using a Nd:YAG laser operating at 1318nm. Such a system would be significantly more efficient than an argon/dye laser. As a totally solid state laser it would also be much smaller and more reliable than the presently used argon/dye lasers. In Phase-I of this project a doubled Nd:YAG laser operating at 659nm will be assembled using a commercially available Nd:YAG laser. Performance of this system will be quantified and preliminary testing in a mouse tumor model will be done with the Medical College of Ohio. In Phase-II a prototype system will be developed for use in precinical and clinical trials of PDT using these new photosensitizers.